A technique for forming a reflective metal film or a distributed Bragg reflector (DBR) on a lower surface of a transparent substrate, such as sapphire, has been widely studied as a study for improving the light extraction efficiency of a light-emitting diode chip, which is a known semiconductor light-emitting device of this type.
FIG. 12 is a cross-sectional view illustrating a known light-emitting diode chip having a distributed Bragg reflector disclosed in Patent Literature 1. FIG. 13 is an enlarged cross-sectional view of the distributed Bragg reflector of FIG. 12.
As illustrated in FIG. 12, a known light-emitting diode chip 100 includes a substrate 101, a buffer layer 102 on the front side of the substrate 101, a light-emitting structural body 103 on the buffer layer 102, a transparent electrode 104 on the light structural body 103, a p-electrode pad 105 on part of the transparent electrode 104, and a n-electrode pad 106 on part of a partly exposed surface of a first conductivity type semiconductor layer 103a of the light structural body 103. The light-emitting diode chip 100 further includes a distributed Bragg reflector 107 on the back side of the substrate 101, a reflective metal layer 108 on the distributed Bragg reflector 107, and a protective layer 109 on the reflective metal layer 108.
The substrate 101 may be any transparent substrate, for example, a sapphire or SiC substrate. The substrate 101 may have a predetermined asperity pattern on an upper surface, that is, over the entire surface, as in a patterned sapphire substrate (PSS). The total area of the chip depends on the area of the substrate 101. The reflection effect increases with the area of the light-emitting diode chip 100.
The light-emitting structural body 103 includes the first conductivity type semiconductor layer 103a, a second conductivity type semiconductor layer 103b, and an active layer 103c, which is disposed between the first conductivity type semiconductor layer 103a and the second conductivity type semiconductor layer 103b. The first conductivity type semiconductor layer 103a and the second conductivity type semiconductor layer 103b are of opposite conductivity types. The first conductivity type may be a n type, and the second conductivity type may be a p type, and vice versa.
The first conductivity type semiconductor layer 103a, the active layer 103c, and the second conductivity type semiconductor layer 103b may be formed of a gallium nitride compound semiconductor substance (Al, In, Ga)N. The composition elements and the component ratio of the active layer 103c are determined such that light having an intended wavelength, for example, ultraviolet light or blue light can be emitted. The first conductivity type semiconductor layer 103a and/or the second conductivity type semiconductor layer 103b may be a monolayer as illustrated in the figure or may have a multilayer structure. The active layer 103c may have a single-quantum-well structure or a multiple-quantum-well structure. The buffer layer 102 between the substrate 101 and the first conductivity type semiconductor layer 103a may be omitted.
The semiconductor layers 103a to 103c may be formed by a metal-organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE) technique and may be patterned in a photolithography and etching process so as to expose part of the first conductivity type semiconductor layer 103a. 
The transparent electrode layer 104 may be formed of ITO or Ni/Au on the second conductivity type semiconductor layer 103b. The transparent electrode layer 104, which has a lower specific resistance than the second conductivity type semiconductor layer 103b, functions to disperse the electric current over the entire chip. The p-electrode pad 105 is disposed on part of the transparent electrode 104, and the n-electrode pad 106 is disposed on part of a partly exposed surface of the first conductivity type semiconductor layer 103a. As illustrated in the figure, the p-electrode pad 105 is electrically connected to the second conductivity type semiconductor layer 103b through the transparent electrode layer 104.
The distributed Bragg reflector 107 is disposed on the lower portion of the substrate 101, that is, the back side of the substrate 101. The distributed Bragg reflector 107 includes a first distributed Bragg reflector 111 and a second distributed Bragg reflector 112.
As illustrated in FIG. 13, the first distributed Bragg reflector 111 includes a plurality of pairs of a first material layer 111a and a second material layer 111b, and the second distributed Bragg reflector 112 includes a plurality of pairs of a third material layer 112a and a fourth material layer 112b. The plurality of pairs of the first material layer 111a and the second material layer 111b may have higher reflectance for light in a red wavelength region, for example, light having a wavelength in the range of 550 or 630 nm than light in a blue wavelength region. The second distributed Bragg reflector 112 may have higher reflectance for light in a blue wavelength region, for example, light having a wavelength of 460 nm than light in a red or green wavelength region. Although the optical thickness of the material layers 111a and 111b of the first distributed Bragg reflector 111 is greater than the optical thickness of the material layers 112a and 112b of the second distributed Bragg reflector 112, the former may be smaller than the latter.
The material and the refractive index of the first material layer 111a may be the same as the material and the refractive index of the third material layer 112a. The material and the refractive index of the second material layer 111b may be the same as the material and the refractive index of the fourth material layer 112b. For example, the first material layer 111a and the third material layer 112a may be formed of a TiO2 film (refractive index n: approximately 2.5), and the second material layer 111b and the fourth material layer 112b may be formed of a SiO2 film (refractive index n: approximately 1.5). In short, 48 layers composed of high-refractive-index films and low-refractive-index films alternately stacked on top of one another have high reflectance in a wide wavelength band.
There is essentially an integral multiple relationship between the optical thickness (refractive index×thickness) of the first material layer 111a and the optical thickness of the second material layer 111b. Preferably, these optical thicknesses may be substantially the same. There is essentially an integral multiple relationship between the optical thickness of the third material layer 112a and the optical thickness of the fourth material layer 112b. Preferably, these optical thicknesses may be substantially the same.
The first material layer 111a may have a higher optical thickness than the third material layer 112a, and the second material layer 111b may have a higher optical thickness than the fourth material layer 112b. The optical thicknesses of the first to fourth material layers 111a, 111b, 112a, and 112b can be controlled by adjusting the refractive index and/or actual thickness of each of the material layers.
The reflective metal layer 108, such as Al, Ag, or Rh, and the protective layer 109 for protecting the distributed Bragg reflector 107 may be disposed on the lower portion of the distributed Bragg reflector 107. The protective layer 109 may be a metal layer selected from Ti, Cr, Ni, Pt, Ta, and Au, or may be formed of an alloy thereof. The reflective metal layer 108 or the protective layer 109 can protect the distributed Bragg reflector 107 from external impact and contamination. For example, the reflective metal layer 108 or the protective layer 109 can prevent the distributed Bragg reflector 107 from being deformed by a substance, such as an adhesive, during mounting of the light-emitting diode chip 100 in a light-emitting diode package.
The reflective metal layer 108 can reflect light passing through the distributed Bragg reflector 107 and can therefore relatively decrease the thickness of the distributed Bragg reflector 107. The distributed Bragg reflector 107 has a relatively high reflectance but may transmit visible light in a long-wavelength region at high incident angles. Thus, the reflective metal layer 108 disposed on the lower portion of the distributed Bragg reflector 107 can reflect light passing through the distributed Bragg reflector 107 and further improve light-use efficiency.
An arrangement of the first distributed Bragg reflector 111 disposed closer to the substrate 101 than the second distributed Bragg reflector 112 can have smaller optical loss in the distributed Bragg reflector 107 than an arrangement of the first distributed Bragg reflector 111 disposed more distant from the substrate 101 than the second distributed Bragg reflector 112.
The light-emitting diode chip 100 having such a structure is a known semiconductor light-emitting device of a face-up light emission type and can emit light from the active layer 103c entirely upward through the p-electrode pad 105 and the n-electrode pad 106.